A variable star, as its name suggests, is a star whose magnitude varies intrinsically, in contrast to eclipsing binaries whose magnitude varies as a result of one star in the binary system eclipsing the other. True variables are one of five types, namely Mira stars, semiregular stars, cepheids, eruptive variables and, finally, cataclysmic variables. Minimum to maximum magnitude can range from days to many months with some variables displaying irregular periods.

A popular method for the study of variable stars, particularly short-term variables, is by the use of the technique known as "differential photometry". Rather than measure the (variable) magnitude of a variable star on an absolute scale, measurements are made over time relative to one or more non-variable star(s) and these differences are then plotted so as to study and illustrate the relative or differential change in magnitude. Due to the very large number of variables stars, the field of differential photometry represents one of the key fields in astronomy whereby the amateur astronomer can make a meaningful and long-lasting contribution to both science and astronomy.

More recently, the search for extrasolar planets (over 710 discovered so far) has identified yet another interesting application for the practice of differential photometry whereby the minute drops in magnitude of a star hosting an exoplanet are studied. Further details for the interested party are available here.

Note: The light curve for exoplanet HAT-P-20b in Gemini depicted below is one of the latest transiting exoplanets, having being announced in Aug/2010 along with three other exoplanet finds, and represents the twentieth discovery by the Hungarian-based HATNet Project team. HAT-P-20b is characterized with a whopping mass 7.246 times that of Jupiter while its radius is equivalent to 0.867 Jupiter radii, thus making this exoplanet one of the densest and most massive "hot Jupiter" finds to-date. HAT-P-20b requires 110.88 minutes to transit its parent star at a depth of 20.4 millimag or 2.04%. It has a circular orbit with a period of 2.875317 and receives below average flux from its (small) host star. The parent star, GSC 1910:0239, is a K3 (binary) dwarf estimated to have a mass of 0.76 solar masses, a radius equivalent to 0.69 solar radii, a temperature of 4,595° K and to lie at a distance of 228 light-years away with a visual magnitude of 11.339. Further details regarding HAT-P-20 and HAT-P-20b are available in the paper published by the discovery team led by Gaspar Bakos et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 1914:617 (mag 11.6) and GSC 1914:590 (mag 11.4) respectively.